A stacked piezoelectric element generally comprises a ceramic stack obtained by alternately stacking piezoelectric layers comprising a piezoelectric material and internal electrode layers having electrical conductivity, and this stacked piezoelectric element is constructed such that a pair of side surface electrodes electrically connected to respective side surfaces of the ceramic stack are provided, a pair of take-out electrodes are joined to respective side surface electrodes by using, for example, an electrically conducting adhesive containing a resin, and piezoelectric displacement is generated due to the driving voltage applied between the paired take-out electrodes electrically connected to respective side surface electrodes.
In such a stacked piezoelectric element, an electrically conducting adhesive having flexibility is used to cope with the displacement of the piezoelectric layer generated during driving but, as the stacked piezoelectric element is used for a long time at a high temperature, troubles such as cracking or separation occur in the electrically conducting adhesive due to displacement of the piezoelectric layer or thermal curing of the resin, and this gives rise to a problem that electrical conduction failure occurs. Therefore, a stacked piezoelectric element having excellent reliability and durability, and usable for a long period of time in a severe environment, is required.
In order to solve these problems, for example, Japanese Unexamined Patent Publication (Kokai) No. 2000-77733 discloses a method of forming a stress-relieving layer on the side surface of a ceramic stack and joining a take-out electrode to the side surface of the stress-relieving layer by using an electrically conducting adhesive containing a resin. However, the strength of the stress-relieving layer is low and therefore, cracks readily occur in the stress-relieving layer. Also, the joined portion between the stress-relieving layer and the take-out electrode is insufficient in adhesion and readily causes separation.
Japanese Unexamined Patent Publication (Kokai) No. 2001-284668 discloses a method of processing an insulating groove in the end part of the internal electrode layer, filling an insulating resin therein, and joining a take-out electrode to the side surface of the ceramic stack by using an electrically conducting adhesive containing a heat-curable resin. However, in this method, similarly to the above, troubles such as cracking or separation occur due to thermal curing or the like of the resin.
The present invention has been made by taking account of problems in conventional techniques, and an object of the present invention is to provide a stacked piezoelectric element with excellent reliability and durability, a production method thereof and an electrically conducting adhesive.